Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.
During operation, wind impacts the rotor blades and the blades transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft. The low-speed shaft is configured to drive the gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed. The high-speed shaft is generally rotatably coupled to a generator so as to rotatably drive a generator rotor. As such, a rotating magnetic field may be induced by the generator rotor and a voltage may be induced within a generator stator that is magnetically coupled to the generator rotor. The associated electrical power can be transmitted to a main transformer that is typically connected to a power grid via a grid breaker. Thus, the main transformer steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to the power grid.
In many wind turbines, the generator rotor may be electrically coupled to a bi-directional power converter that includes a rotor-side converter joined to a line-side converter via a regulated DC link. More specifically, some wind turbines, such as wind-driven doubly-fed induction generator (DFIG) systems or full power conversion systems, may include a power converter with an AC-DC-AC topology. Standard power converters typically include a bridge circuit, a power filter, and an optional crowbar circuit. The bridge circuit typically includes a plurality of cells, for example, one or more power switching elements and/or one or more diodes.
The magnitude of the DC link voltage, i.e. the DC link voltage set point, is controlled by a boost control algorithm, which must by definition be kept at a level equal to the higher of the peak value of the rotor and mains voltage, plus some additional margin to allow for an ability to force current through the line and rotor impedance. Further, the maximum allowable DC link voltage is determined by the design of the rotor-side and line-side converters. More specifically, the DC link voltage can be governed by selection of the power switching device type and ratings, selection of the DC link capacitance type and ratings, parasitic elements (e.g. stray inductance), and the operation of the gate drivers that govern the switching of the power devices and consequently the transient overshoot voltage seen by the power switching devices.
The steady state DC link operating voltage set point impacts a number of items including but not limited to the maximum magnitude of fundamental AC voltage available at the rotor and line sides of the converter, the semiconductor losses, and the failure rate of the switching devices. In order to maximize the operating voltage of the DFIG, the steady state DC link operating voltage set point should be set as high as possible. Conversely, to obtain robustness in regards to grid voltage capability, a margin is required between the steady state DC link operating voltage set point and the maximum allowable instantaneous DC link voltage to avoid component failure, thereby requiring a lower steady state DC link voltage set point.
Accordingly, a converter DC link control methodology that allows for a converter design that can operate at a higher steady state DC link operating voltage set point, while minimizing losses, retaining robustness against grid events and minimizing negative effect on power switching device reliability would be advantageous.